Improved Calibration of Cutthroat Flumes
An analysis of laboratory data for a 0.914-m (3-ft) length Cutthroat flume with four different throat widths is presented. Significant differences with previously published calibration parameters were found. Special attention was given to non-hydrostatic pressures at the upstream and downstream piezometer taps, and the variations are presented in a series of three-dimensional plots. Variation in relative pressure at different tap heights was observed and was concluded to represent a shift from a concave to a convex flow profile between the 0.305- and 0.203-m throat widths. This significant alteration in the flow profile correlates with a sharp change in the nf calibration parameter, describing a non-linear relationship with flume throat width. Alternative equation forms were explored in an attempt to increase the predictive accuracy of the calculated flow rates for free- and submerged-flow regimes. The alternative equations showed a decrease in the percent error, in the submerged-flow regime, by more than 50%. Transition submergence was observed to vary not only due to flume size, but also to flow rate. An empirically fitted equation was developed to calculate the transition submergence as a function of throat width and flow rate. In addition, a separate calibration for free-flow parameters was defined based on measurements from an upstream point gauge. The flow measurement accuracy of existing Cutthroat flumes can be significantly increased, especially for submerged-flow regimes.
Weber, R.C., Merkley, G.P., Skogerboe, G.V. et al. Irrig Sci (2007) 25: 361. https://doi.org/10.1007/s00271-006-0052-x